Well, it could be about Harry Potter finding a way to enter Tom Riddle's world in the fanfiction. Maybe he's on a mission to change Tom's path to the dark side.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
Perhaps Harry is betrayed by the wizarding world as a whole. The Ministry of Magic wrongly accuses him of a serious crime. With no one to turn to, Voldemort approaches Harry. Voldemort shows Harry a world where he can be in control, instead of being at the mercy of those who have turned against him. So, Harry decides to join Voldemort out of desperation and a thirst for revenge against those who betrayed him.
It could be Voldemort. Even if it's a fanfiction where Harry joins Tom, Voldemort's future self might still be a threat and oppose Harry's attempts to change Tom.
Perhaps Harry joins Tom's eventual lover in order to uncover some secrets that Tom has been hiding. They could start off as distrustful of each other but as they work together, they find common ground. The plot might involve them traveling to different magical places and facing various challenges together while trying to figure out Tom's true nature.
There could be other Hogwarts students like Hermione and Ron. Hermione might offer her intelligence and knowledge to help Harry and Tom's lover in their journey. Ron could be there for comic relief and also provide some muscle when needed. And of course, there might be some of Tom's followers or enemies that play important roles in the story.
The wizarding world would be in shock. Harry has always been seen as a symbol of hope and the one who could defeat Voldemort (Tom). People would feel betrayed, especially his friends like Hermione and Ron. The Ministry of Magic would likely see Harry as a new threat and start devising plans to stop him. There would be a lot of fear and uncertainty among the wizarding population.
The Ministry of Magic could be the ones to betray Harry. They might try to use Harry as a pawn in their political games, and when Harry realizes this, he loses faith in the so - called good side. Voldemort then takes advantage of Harry's disillusionment and convinces him to join.
Well, perhaps Percy is betrayed by his friends or allies who he thought he could trust. This betrayal could be due to jealousy, a misunderstanding, or someone's hidden agenda. As a result, Percy feels so hurt and angry that he decides to join the Hunt. In the Hunt, he might find a new sense of purpose, new friends who understand his pain, and he could start to train hard to become a more powerful fighter. He may also seek revenge against those who betrayed him.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.